Rough vacuum pump using bulk getter material

ABSTRACT

A bulk getter-pump, consisting primarily of large beds of heated getter-material for use in pumping down a vacuum chamber to a rough vacuum. The pump is designed for applications now are served by turbo, cryo, diffusion, and ion pumps. The pump consists of a meshed cage filled with bulk getter-material pellets, which cage is housed in a housing coupled to a conduit of a vacuum chamber, so that the bulk getter-material is exposed to the interior of the vacuum chamber. In use, a roughing pump is first used to bring the chamber down to a pressure of about 2 torr, and then the bulk getter-pump of the invention is operatively coupled to the chamber for sorbing gases, in order to reach a desired vacuum.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of application Ser.No. 07/747,667, filed on Aug. 20, 1991.

BACKGROUND OF THE INVENTION

Most industrial and semiconductor vacuum processes are performed in highvacuum environments, where pressures are between 10⁻⁴ to 10⁻⁸ torr.Ultra-high vacuums, where pressures are below 10⁻⁸ torr, often use thetechnique called "Getter Pumping". The present invention has advancedthe use of the Getter pumping technique into rough-vacuum applications,with significant performance-advantages over conventional pumpingtechniques.

Gettering is a process that pumps or purifies gases by chemical reactionwith an active metal material. For example, oxygen is pumped by forminga metallic oxide. Getter material will not pump inert gases such ashelium, argon, krypton, neon or xenon. Evaporable getters are presentlyin use in the vacuum industry for pumping. In this case, the gettermaterial, usually titanium, is deposited by evaporation onto asubstrate, such as the lid of a vacuum chamber, as a thin film. The thinfilm is quickly used up by reaction, and must be constantly replaced, inthe pressure range of between 10⁻³ to 10⁻⁷ torr. In fact, it isdifficult to replace the film fast enough to use in this pressure range,so this process is generally only used at ultrahigh vacuum (below 10⁻⁷torr), where there are fewer molecules requiring pumping.

Non-evaporable getters are materials that can be used by heating a solidmaterial to temperatures high enough to make it react with the activegases to be pumped. In general, the gases will react on the surface ofthe getter material, and then slowly migrate into the bulk or body ofthe getter material. The temperature of a given getter material willcontrol both the rate of reaction and the speed at which the migrationoccurs. A new charge of getter will be covered with reacted material andwill require "activation" before it will pump efficiently. In mostcases, this requires a short heating cycle that is at a highertemperature than the operating temperature. The pumping speed of thepump is dependent upon the amount of getter material available forreaction, and the amount of surface area available for reaction.

As stated above, the use of getter-materials as vacuum-pumping vehiclesis well-known. The "gettering" process has been around since the earlydays of the electron tube industry. Materials that react withchemically-active gases to produce low, vapor-pressure compounds wereplaced in electron tubes to "get" the gases. The term has survived, asdid the terms "getters" and "gettering".

Getter-materials are used in many products and processes where one needsto maintain a vacuum against small gas-loads. Sputter-ion pumps andtitanium sublimation-pumps use getter-materials in their operation.Strip-mounted films of getter-material also are used as hydrogen pumpsin accelerators. The common thread that runs through these getteringsystems is that each is a relatively low-throughput device that isideally suited for clean, ultra-high vacuum processors, where gas-loadsare low.

As stated above, getter-pumps may be divided into two basic types:Deposited film (evaporable) or stable state (non-evaporable).Deposited-film, evaporable getters are the more common of the two. Asexplained above, thin films of getter-material are deposited on hostsurfaces, such as a chamber wall, where the gettering action takesplace. These surfaces are at room temperature, or are cooled below roomtemperature. The deposited (getter) films are formed by sputtering, asin sputter-ion pumps, or by evaporation, as in sublimation pumps.Titanium is the most commonly deposited getter material. Unfortunately,these films are quickly used up by reaction with the pumped activegases, and must be continually renewed. This means that the gasload thefilm is expected to pump is proportional to the rate of renewal neededfor the film material. Although deposited films are perfectly capable ofpumping a system down to high vacuum from roughing pressure, they havedifficulty in meeting a steady gas-load at these pressures. These filmsalso have lower pumping speeds at conventional roughing pressures, sothey are difficult and time-consuming to use during the first part of apump-down cycle. Deposited films do not reach their bestperformance-levels until ultra-high vacuum (UHV) levels are reached. Inaddition, they require a significant surface area upon which to bedeposited. Although deposited-film getters are clean, their lowthroughput in high vacuum is compounded by their need for large hostsurfaces. This usually results in peeling and powdering of the exhaustedfilms, necessitating frequent cleaning of the pump.

The second type, steady-state, or non-evaporable, stable getter pumpsuse the same pumping mechanism as the thin-film getters, in that theyreact with the active gases to be pumped. But their similarity stopsthere. Steady-state getter material remain as solid forms thatcontinually sorb the gases. Normally, these materials are heated duringoperation. Heat helps diffuse the pumped, active gases into the bulk ofthe getter-material, which then continually exposes freshgetter-material surfaces. Steady-state getter-materials are commerciallyavailable in strip form, where a getter-film is bonded to a supportstrip, or in bulk forms, such as pills, pellets, or chunks. As in allgettering systems, a solid-state getter-material has a finite ability tosorb gases before it becomes saturated.

As stated before, most commercial vacuum processes only require highvacuum, not ultra-high vacuum, and, therefore have not used thegetter-pumping method, because of the disadvantages summarized above.These high-vacuum (HV) commercial processes require short pump-downtimes and repeated pump-down cycles between process-loads. In manyprocesses, however, fast pump-down is not enough. Cleanliness of thepumping process also is vital, as more stringent processes aredeveloped.

Conventional, non-gettering, pumping techniques typically employ anoil-sealed mechanical pump, which cannot easily reach roughing pressuresbelow several millitorr. The amount of gas that any getter-pumpingsystem would have to sorb at these relatively-high pressures wouldexhaust its ability to pump, if it had to pump down repeatedly fromthese high pressures. An oil-sealed, mechanical pump requires anadditional high-vacuum pump, such as a turbomolecular pump, to reachlower pressures before any gettering pump could be employed.Molecular-drag pump technology changed this by not only providingcleaner roughing, but by also allowing roughing pressures of 10⁻⁴ to10⁻⁶ torr to be routinely achieved. Roughing to these pressures hasopened a new application for getter pumping. Experiments with a small,strip getter-pump demonstrated that one can easily and quickly evacuatea chamber when roughing pressures are reduced below 10⁻⁴ torr. However,there is a problem with using a molecular-drag pump/getter strip pumpsystem for industrial processes. When the pump has to re-evacuate thechamber shortly after it has been opened to air between process cycles,the throughput becomes limited. This limitation is traceable to theamount of surface area of getter material available for pumping. Whenthe pump meets a steady-gas load, the getter surface becomes coveredwith reacted gas, which diffuses into the bulk of the material at a rategoverned by its composition and temperature. If the surfaces are inequilibrium with a small gas load, the surface will not be able torecover its full pumping speed quickly enough to deal with a higher gasload when the system is opened to air and roughed down quickly.

SUMMARY OF THE INVENTION

It is the primary objective of the present invention to provide agettering pump that may be used effectively and continuously in arough-vacuum environment in the pressure range of between 2 torr and10⁻⁵ torr.

It is another objective of the present invention to provide such agettering pump for use in rough-vacuum applications that will sorb gasesat a fast enough rate so as to meet the steady-state processrequirements associated with any rough-vacuum application.

According to the present invention, a gettering pump is provided for usein rough-vacuum applications without the drawbacks and adversecharacteristics described above.

A bulk getter-pump, consisting primarily of large beds of heatedgetter-material, has been specifically developed for processes, such assputtering and evaporation, that require high pumping speeds andthroughput in a rough-vacuum environment. These bulk getter-pumps arecompletely solid-state devices. There is no possible oil contamination,no vibration, and no sound. They produce no magnetic fields, and are notaffected by magnetic fields. They are designed for applications that noware served by turbo, cryo, diffusion, and ion pumps. The main thrust ofthe invention is to provide pumping of large gas loads in a rough-vacuumregion.

The bulk getter-pump of the invention may be used in uniqueapplications, such as a sputtering process, which can be performedwithout the need to continually flow argon gas. If the getter-materialis kept hot, the pump of the invention will pump any contaminant gasesthat are introduced with the argon or released by the process; however,the argon will not be pumped away, so flow meters and flow controllersare not required.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood with reference to theaccompanying drawing, wherein:

FIG. 1 is a front, isometric view of the gettering-pump for use inrough-vacuum applications according to the invention;

FIG. 2 is a rear, isometric view of the gettering-pump for use inrough-vacuum applications according to the invention;

FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 1;

FIG. 4 is a schematic view showing the gettering pump of the inventionin combination with a mechanical, roughing pump;

FIG. 5 is a cross-sectional, side view showing a modification of thegetter pump of the invention in which the external heater is receivedwithin a well formed in the cage of the pump holding the bulkgetter-material.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings in greater detail, the getter-pump forrough-vacuum applications is indicated generally by reference numeral10. The getter-pump 10 has a main, sealed, outer housing 12 defining alower cylindrically-shaped portion 14 and an upper, smaller-diameter,cylindrically-shaped, clamping-ring portion 16. The housing 12 mountstherein a getter-pump body housing 20 that is also cylindrical in shape.The pump-body housing 20 is mounted in the main housing 12 by anintegral, annular, mounting flange-member 22 that is removably securedto the interior of the upper clamping-ring portion 16 via a clamp 24.The clamping-ring portion 16 is a split ring with the clamp 24 holdingthe ends thereof together for holding the ring fast to the flange-member22. The flange-member 22 has a pair of oppositely-disposed holes 26 forthreadingly-receiving a pair of bolts 28, whereby another, mating,annular, 23/4 inch, Conflat flange-member 30 of a conduit section 32 maybe secured to the flange-member 22 in a face-to-face manner, as clearlyshown in FIG. 3. An annular seal 30' provides a sealed connection. Withthe two flanges secured to each other, the interior of the conduitsection 32 is in coaxial, linear alignment with the interior of thepump-body housing 20. The conduit-section 32 is coupled at its other,distal end (not shown) to a vacuum chamber or vessel which is to bepumped down to a rough vacuum in the range between 2 torr and 10⁻⁵ torrby the pump 10 of the invention, after such chamber or vessel has beeninitially pumped down by a conventional roughing pump, as describedbelow in greater detail. The interior volume of the housing 12 isprovided with heat-insulating material 23 that surrounds the pump-bodyhousing 20. Positioned within the pump-body housing 20 is a removable,circular cross-sectioned, screen-cage 38 made of wire-mesh. The cage 38has a central, tubular support member 40 that is surrounded by anannular, getter-material storage, volume-section 42 that is filled withgetter-material 44 in bulk form, such as pellets, pills, and the like.The particular type of getter-material depends upon the gas to be sorbedthereby, which is well-known in the art. The height of the cage 38 issubstantially the same as the height of the pump-body housing 20. Thescreen mesh and the hollow tubular central section 40 allow for thegreatest possible surface-area exposure of the bulk getter-material, soas to increase the getter-pumping of the pump, and its sorption ofgases, in order to create and sustain a rough vacuum in a vacuum vesselor chamber. Operatively associated with the getter-pump body 20 is anannular band-heater device defining a series of annular coil-segments 48by which the pump-body housing 20 may be heated, whereby the bulkgetter-material 44 may be kept at a desired temperature. The coils arepowered via a power cord 50 passing through an opening 52 formed in aportion of the annular surface of the lower housing portion 14. Astand-alone power-unit 37, as shown in FIG. 2, may be provided forsupplying the power to the coils and for controlling the "on" and "off"states of the heater.

The getter-materials used are porous and pellet-shaped, and are aboutthe size of small aspirin tablets, so as to increase the working surfacearea. They may also contain a bed of pellets, pills or chunk gettermaterials, or a mixture of these getter-materials. This large amount ofgetter-material allows a correspondingly large amount of gas to bepumped in total, which results in a long use-period before thegetter-material is totally reacted and has to be replaced.

In one version of the device 10, getter-pellets are placed in a 3.8 cm.diameter cage-cylinder 42 with the upper end portion of the pump-bodyhousing 20 being welded to the flange 22. In a process mode, thegetter-material 44 in the bulk pump is heated to 280 degrees C. Since itis impractical to cool down the getter-material before opening thechamber to air, a valve isolates the pump from the chamber betweenprocess cycles, as described below in greater detail. New pumps requirean activation cycle to remove the reacted layer on the outer surfaces ofthe getter-material. Initial heating of the new pump to about 500degrees C. for 30 to 60 minute diffuses the reacted surface into thebulk of the getter pellets. This initial heating process can be repeatedwhenever the pump 20 loses speed over time due to surface saturation,but this is only needed a few times during its lifetime. Although thebulk getter pump 20 requires no maintenance, the getter-material 44 hasa finite lifetime. In normal operation, the getter-material lastsapproximately six months to a year. When pump performance falls off, andreactivation via heating to, for example, 500 degrees C., no longerworks, the getter-charge 44 is merely dumped out of the pump housing 20,and is replaced with a new, fresh charge.

The pump 20 is inherently clean and free of any vibration and noise. Itis immune to particle ingestion problems. Its small size, low initialcost, low operating cost, and minimum of maintenance make it an idealprocess pump for rough vacuum applications. The pump 20, being a roughvacuum pump, requires pre-pumping to at least 2 torr, before it isvalved into operation. The pump 10, in its capacity as a roughing pump,is capable of pumping to pressures below 10⁻⁵ torr. In the smallestversion of the pump 10, the getter-charge is contained within a 11/2 in.diameter, tubular, stainless-steel housing that is provided with a metalsealed flange to allow de-mountable connection to a vacuum system orvacuum vale.

FIG. 4 is a schematic diagram showing the combination of a bulk-getterpump in combination with a mechanical roughing pump 80. The roughingpump 80 may be 4-stage diaphragm pumping unit manufactured by Danielson& Associates of Lisle, Ill., called "BARODYN". The "BARODYN" pump isessentially a roughing pump, since it is used in a different, butoverlapping, pressure range than the pump 10. The pump 80 reduces thepressure in a vessel 82 to about 2 torr. At this pressure, the diaphragmpump portion 80 is valved off via valve 84 and the pump 10 is valved invia valve 86. The pump 10 then reduces the pressure to the pump'sultimate. In the case of the pump-down from air, the ultimate pressurewill be limited to 10-30 millitorr. This is due to the percentage ofargon (air contains 0.9% by volume of argon) that is left in the vesselbeing evacuated when the bulk getter-pump 10 is valved in. The pump 10will not pump argon at all. If a chamber is filled with active gas suchas nitrogen, the ultimate pressure will be 10⁻⁶ torr, or lower.

Since the combination of FIG. 4 uses the pump 10, it is capable of usingdifferent getter-material mixtures, with the operating temperatures andultimate pressures achieved dependent upon the actual getter material ormaterials used. In general, though, the activation temperature will beabout 500 degrees C., while the operating temperature will be higherthan in the pump 10. Temperatures up to 500 degrees C. can be useddepending upon the application.

The pump operates as follows. Before operation, all valves are closed.When the pump is turned on, the diaphragm portion 80 starts, and thevalve to the diaphragm portion opens. The pressure is reduced in thevessel 82 until a pressure switch 88 senses its preset pressure, whichis typically 2 torr. Then, the valve 84 to the diaphragm portion closes,and the diaphragm portion is turned off. The valve 86 to the bulkgetter-pump portion 10 is opened, and the getter-material reduces thepressure to the pump's ultimate. The getter material in the pump 10 canbe exposed to air only when it is cold. Therefore, a pump used in aprocess environment should be valved, so that it can be maintained atoperating temperature at all times, avoiding time-consuming heat-up,cool-down cycles.

A smaller version 50 of the device is shown in FIG. 5, and has a pump 52with a body housing 53 in which is received a cage 54 like that of thedevice 10. A main outer housing 56 surrounds the pump 52 and is providedwith a connecting flange 60 that, like the flange 22 of the device 10,connects to a similar, mating flange of a conduit leading to a vacuumchamber or vessel to be pumped down and sustained at rough vacuum. Theouter main housing 56 is preferably provided with holes or openingsthereabout for air circulation. Owing to the relatively smaller size ofthe device 50 as compared with the device 10, no insulating material isneeded, and the large, annular region for storing the insulatingmaterial is not required. Thus, instead of an annular coil or bandheater of the device 10, the device 50 has a rectilinear-shaped heaterunit 60 that is received in a central well 62 formed in the cage 54,which heater extends downwardly and out of the pump-housing 53, as seenin FIG. 5. The heater unit 60 also has a power cord (not shown)extending out of the main housing, as in the device 10. A control unitsimilar to the unit 37 may also be used for the device 50.

While a specific embodiment of the invention has been shown anddescribed, it is to be understood that numerous changes andmodifications may be made therein without departing from the scope,spirit and intent of the invention as set forth in the appended claims.

What I claim is:
 1. In a vacuum chamber that is to be partiallyevacuated to a rough vacuum of between approximately 2 torr and 10⁻⁵torr, the improvement comprising:a getter pump operatively coupled tosaid vacuum chamber for pumping down said vacuum chamber to a roughvacuum of between 2 torr and 10⁻⁵ torr; a mechanical roughing pump forinitially pumping down said vacuum chamber; and valve means foralternately coupling said bulk getter-material pump and said mechanicalroughing pump to said vacuum chamber to be pumped down; said valve meansbeing operatively connected between said vacuum chamber to be pumpeddown and said pumps.
 2. In a vacuum chamber that is to be partiallyevacuated to a rough vacuum of between approximately 2 torr and 10⁻⁵torr, the improvement comprising:a getter pump operatively coupled tosaid vacuum chamber for pumping down said vacuum chamber to a roughvacuum of between 2 torr and 10⁻⁵ torr; said getter pump comprising: abulk getter-material pump comprising a pump-body housing mounted, saidpump-body housing comprising an interior; a cage-element mounted in saidinterior of said pump-body housing; a supply of bulk getter-material insaid cage-element, said bulk getter-material comprising a plurality ofseparate getter-pellets; means for fluidly coupling said bulkgetter-material pump to said vacuum chamber to be pumped down; andheating means operatively associated with said bulk getter-material pumpfor heating said supply of bulk-getter material.
 3. The improvementaccording to claim 2, further comprising an outer housing comprisinginsulating material therein about said bulk getter-material pump forretaining the heat from said heating means.
 4. The improvement accordingto claim 2, wherein said cage element is removably mounted in saidinterior for subsequent removal in order to place a new, fresh charge ofbulk getter-material therein, said cage element comprising a centralhollow cage-member, and an annular, hollow cage-member about saidcentral cage-member; said bulk getter-material being stored in saidannular, hollow cage-member.
 5. The improvement according to claim 2,wherein said heating means comprises an exterior, annular coil-heatingmeans circumferentially surrounding a portion of the circumference ofsaid pump-body housing, and electrical power means connected to saidcoil-heating means for supplying electrical current thereto.
 6. Theimprovement according to claim 2, wherein said cage-element serves as aheater-well receptacle; said heating means comprising a heaterpositioned in said heater-well receptacle; and power means exterior ofsaid heater for powering said heater.
 7. The improvement according toclaim 2, wherein said pump-body housing comprises an open entrance mouthleading to said interior thereof; said means for fluidly couplingcomprising an annular mounting flange about said pump-body housing atsaid entrance mouth; said vacuum chamber comprising a mounting flangeand a conduit leading to the interior of said vacuum chamber; saidannular flange of said means for fluidly coupling being coupled to saidmounting flange of said vacuum chamber.
 8. The improvement according toclaim 7, further comprising an outer housing comprising a first, lower,larger-diameter portion and a second upper, smaller-diameter portion;said annular flange being peripherally encircled by a portion of saidsecond portion; said second portion comprising clamping means forsecuring said annular flange therein.